SIR-C/X-SAR (Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar) was a joint
project of the National Aeronautics and Space Administration (NASA), the German Space
Agency (DARA) and the Italian Space Agency (ASI). An imaging radar system launched aboard
the NASA Space Shuttle twice in 1994, SIR-C/X- SAR's unique contributions to Earth
observation and monitoring were its capability to measure, from space, the radar signature
of the surface at three different wavelengths and make measurements for different
polarizations at two of those wavelengths. SIR-C image data will help scientists
understand the physics behind some of the phenomena seen in radar images at just one
wavelength/polarization, such as those produced by SeaSAT. Investigators on the
SIR-C/X-SAR Science team will use the radar image data to make measurements of: Vegetation
type, extent and deforestation; Soil moisture content; Ocean dynamics, wave and surface
wind speeds and directions; Volcanism and tectonic activity; Soil erosion and
desertification.

X-SAR provides single frequency (3.1 cm), single polarization (vertical) data and SIR-C
provides multi-frequency, multi-polarization radar data. The SIR-C antenna was composed of
two planar arrays, one for L-band (23.5 cm) and one for C-band (5.8 cm). Each array was
composed of a uniform grid of dual- polarized microstrip antenna radiators, with each
polarization port fed by a separate corporate feed network. These vertically- and
horizontally-polarized transmitted waves were received on two separate channels so that
images of the magnitude of radar backscatter were acquired in four polarization
combinations: HH (Horizontally-transmitted, Horizontally-received), VV
(Vertically-transmitted, Vertically-received), HV, and VH. Data was also acquired on the
relative phase difference between the HH, VV, VH, and HV returns. By properly phasing the
array, the beam could be electronically steered in the range direction up to 23 degrees from
the nominal 40 degrees off nadir position without physically moving the antenna. The X-SAR
antenna was mechanically tilted to angles between 15 and 60 degrees.

Both SIR-C and X-SAR could be operated as either stand alone radars or together. Roll and
yaw maneuvers of the shuttle will allow data to be acquired on either side of the shuttle
nadir (ground) track. The width of the imaged swath on the ground varies from 15 to 90
kilometers (9 to 56 miles) depending on the orientation of the antenna beams and the
operational mode. The data collected was processed into images with resolution
selectable from 10 to 200 meters.

Two types of image products were produced by the ground data processing systems: the
Survey product and the Precision (Standard) product. JPL was originally responsible for Survey and Precision processing of the SIR-C data, while ASI was responsible for Survey processing of
the X-SAR data. Precessing processing of the X-SAR data was shared between the ASI and the
DLR. The Precision product is a frame image of a subset of the data take (swath). Only a
small percentage of the acquired data has been processed into Precision products.

All science data were processed into Survey products. The Survey product is
intended as a "quick look" browsing tool for viewing the areas imaged by
SIR-C/X-SAR. It is not designed to be used for quantitative scientific analysis. The
Survey product is a strip image of the entire data take (swath) with the following
properties: